Ion implantation is a semiconductor doping process whereby a plurality of dopant atoms are first ionized, then accelerated to velocities sufficient to penetrate the semiconductor surface and deposit within. Well known are semiconductors that can be altered in electrical behavior by the introduction of minute quantities of elemental materials called "dopants". Dopants generally come in either p-type or n-type. P-type dopants (including boron, aluminum, gallium, thallium, indium and/or silicon) produce what is commonly known as hole conductivity while n-type dopants (including phosphorous, arsenic and/or antimony) produce what is commonly known as electron conductivity. The combination of hole and electron regions produces desired devices such as transistors, resistors, diodes, capacitors, etc., which form the basis of semiconductor operation.
An ion implanter is commonly used in the semiconductor industry to introduce dopants into semiconductor substrates. The depth in which dopant ions are introduced into the substrate increases as acceleration voltage of the implanter increases. Moreover, the total number of ions introduced is proportional to the beam current and implant time of the implanter.
Due to the growing popularity of sub-0.25 .mu.m, low energy (or low current) implanters have become a mainstay in integrated circuit manufacturing. A low energy implanter is required to form ultra shallow junctions in the semiconductor substrates that are necessary for enhanced performance in these high density sub-0.25 .mu.m circuits.
Integrated circuits are generally formed by interconnecting numerous individual devices set forth by dopant implantation. A single wafer may contain several thousand devices which are diced and individually packaged as a single monolithic circuit. It is important that the doping process be accurately presented to the semiconductor area in order to ensure the monolithic circuit operates according to target parameters. If doping does not bring about such operation, then the corresponding yields may be drastically reduced, thereby adding to the cost of manufacturing. Important factors relating to accurate doping include: (i) the need to control the number of doping ions introduced (dosimetry) in a semiconductor substrate, and (ii) the need to control the depth or concentration profile of dopant placed into the semiconductor substrate. These factors are even more important for sub-0.25 .mu.m devices and therefore must be closely monitor.
The conventional method of monitoring the number of doping ions introduced in a semiconductor substrate is by integrating the total current collected by a Faraday cup located behind the semiconductor substrate in the implanter. The assumption is that during a particular portion of the implant cycle, the semiconductor wafer is positioned out of the beam and the beam current will fall onto the Faraday cup. As such, the measured charge quantity accurately reflects the amount of dopant ions delivered to the semiconductor substrate.
One drawbacks of using low energy ion implantation is that the beam tends to travel in a divergent pattern instead of a straight-line pattern; the lower the energy, the higher the divergence. Portions of dopant ions from a divergent, low energy beam may not strike the substrate and yet will be collected by the Faraday cup. Hence, the divergent pattern of a low energy beam reduces the accuracy in using the Faraday cup to determine the number of doping ions introduced in the semiconductor substrate.
Other quantitative techniques that are commonly used to determine the amount of dopant ions delivered to a semiconductor substrate include secondary ion mass spectroscopy (SIMS) and spreading resistance probe (SRP). In SIMS techniques, a mass spectrometer is used to identify the elemental compositions of small pieces of materials dislodged from the surfaces of a semiconductor substrate by ion bombardment. To determine elemental compositions in regions of a semiconductor substrate below the surface, such regions must first be exposed. It has been found that the SIMS techniques are not very accurate in determining the amount of dopant ions delivered to a semiconductor substrate for sub-0.25 .mu.m devices. The problem is due to the difficulty in removing a very small and precise amount of material from the top surface of the semiconductor substrate in order to expose the underlying shallow junctions that have a depth in the range of only about 10 to 100 angstroms. Other drawbacks with the SIMS techniques are that they are very tedious and expensive. In using SRP techniques to determine the doping profile of a semiconductor substrate, the semiconductor substrate is first angle lapped (tapered) and then subject to spreading resistance (SR) measurements along its length using two probes. From the angle of the taper, the depth as a function of distance from the surface of the semiconductor substrate can be found. The doping profile is then computed based on the SR measurement using some well-known equations that govern the relationships between the resistivity, the SR, and the effective electrical contact radius between the probes and the surface of the semiconductor substrate. The SPR techniques, however, are not very accurate or reliable for sub-0.25 .mu.m application because of the difficulty in making good electrical contacts to the surface of the semiconductor substrate. Furthermore, just like the SIMS techniques, SPR techniques are tedious and expensive.
A solution, which would provide an accurate, reliable, simple and inexpensive method for determining the dosimetry of a semiconductor substrate with sub-0.25 .mu.m technology, has been long sought but has eluded those skilled in the art. As the semiconductor industry is moving quickly to sub-0.25 .mu.m technologies, it is becoming more pressing that a solution be found.